Thread question

[Matheus Souza]

Hey guys, I was doing an mock from ExamLab and this question appeared:

public class ThreadClass extends Thread{

     public ThreadClass(int x){
          this.x=x;
     }

     static String s="";
     static volatile int x=0;

     public static void doVIP(){
          s=s+x;
          try{
               Thread.sleep(10);
          }catch(Exception e){
               s=s+x;
          }
     }

     public void run(){
          synchronized(s){
               doVIP();
          }
     }

     public static void doDelay(long g){
          try{Thread.sleep(g);}catch(Exception e){}
     }

     public static void main(String args[]){

          ThreadClass tc1=new ThreadClass(2);
          ThreadClass tc2=new ThreadClass(1);
          ThreadClass tc3=new ThreadClass(0);

          tc1.start(); doDelay(100); System.out.print(s);
          tc2.start(); doDelay(100); System.out.print(s);
          tc3.start(); doDelay(100); System.out.print(s);

     }
}

It says that the answer is: 000000

Can anyone explain to me what happened?

Thanks

[Anayonkar Shivalkar]

Matheus Souza wrote:It says that the answer is: 000000

That is correct.

Matheus Souza wrote:Can anyone explain to me what happened?

What exactly didn't you understand? Is it why 6 numbers are printed? Or is it why 0 is printed every time? Or both?

6 numbers were printed because, after execution of 1 thread (just one thread. not first or second - because we cannot guarantee which thread will execute first), length of s would be 1. After execution of 2 threads, length of s would be 2 and after execution of 3 threads length would be 3. Further, we are printing s for 3 times, hence, we get 1+2+3=6 numbers. This modification of s is thread safe because of synchronization(line 20)

Now, what happens to those numbers we passed during object creation (2,1 and 0)? Well, x is volatile. So, every time, a thread would refer copy from main memory (and not thread's local memory), so it will take 0 every time.

I hope this helps.

[rimantas grebliunas]

IMO x variable is static so it's value the same for all objects. The last constructor set x to 0 and then started all threads, thats why result 000000.

[Anayonkar Shivalkar]

rimantas grebliunas wrote:IMO x variable is static so it's value the same for all objects. The last constructor set x to 0 and then started all threads, thats why result 000000.

How did I miss this?

Thanks rimantas grebliunas for the correction.

[Matheus Souza]

Anayonkar Shivalkar wrote:

rimantas grebliunas wrote:IMO x variable is static so it's value the same for all objects. The last constructor set x to 0 and then started all threads, thats why result 000000.

How did I miss this?

Thanks rimantas grebliunas for the correction.

Thanks you guys, I got it. This is a very tricky question, insn't it? =]